As weed heads for retail stores for users 21 and older starting next year, California leaders appear to be learning a lot from the mistakes of other marijuana-legal states.

One lesson is that a blood alcohol–style limit for drivers probably doesn't work. Other states, including Washington, established a DUI limit of "5 ng/ml or more of delta 9-tetrahydrocannabinol" in a suspect's bloodstream. The problem is that this limit does not necessarily correlate to being high. Marijuana users could, for example, have smoked two days ago and still show a 5 ng/ml reading, experts say. Even the AAA Foundation for Traffic Safety concluded last year that "legal limits for marijuana and driving are problematic," according to the Auto Club of Southern California.

Despite previous attempts to establish a blood alcohol–style limit, the Legislature appears to have heard the AAA's message, and lawmakers are headed in a new direction with a bill by Assemblyman Tom Lackey of Palmdale. His successful "trailer" bill, intended to tie up loose ends presented by California voters' passage of Proposition 64, the November initiative that legalized recreational pot, creates a "task force will develop best practices that can be used by law enforcement along with recommendations on legislation by 2021," according to Lackey's office.

"The task force will also test and examine roadside drug testing technology," according to a fact sheet for the legislation.

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Dale Gieringer, state coordinator of California NORML, says the state will essentially study how best to determine if a driver is stoned. The research could take a few years, he said. He and other cannabis advocates believe that standard police field tests for DUI, supplemented with common-sense drug recognition, will be sufficient until them.

"If they can show in court you were impaired by marijuana, it's DUI," he says. "You were arrested, you looked impaired, you were driving screwy, you smell like marijuana — they can prosecute you for DUI. That's always been the case."

Drug Policy Alliance staff attorney Jolene Forman agrees, saying THC tests are almost meaningless and could end up putting innocent Californians behind bars. "Rather than arbitrarily setting THC limits that tell us nothing about whether a driver is impaired, the cannabis trailer bill language provides Californians with an opportunity to better understand and detect actual impairment, including marijuana impairment," she said via email.

"THC threshold tests only detect whether a person has consumed marijuana sometime in the past several hours, days or even weeks," she said. "Money in the trailer bill will give researchers the opportunity to validate tests that measure whether, and the extent to which, a driver is impaired."

Last week lawmaker Lackey said in a statement, "Utilizing CHP’s unmatched roadway safety expertise to find the right technology and best practices will make sure we hold people accountable who drive high while avoiding falsely accusing those who are not."

Scott Chipman, Southern California chair of the group Citizens Against Legalizing Marijuana, says no drug limit — even blood alcohol — is foolproof; but drawing such a line is necessary.

"Some drinkers are .08 [at the California blood alcohol content limit] and say they're not impaired, but you have to set a limit someplace, and it has to be on the safe side," Chipman says. "Why wouldn't we want to be on the safest side of the issue? We don't want people to even come close to driving impaired."

L.A. Weekly staff writer Dennis Romero has worked on staff at several magazines and newspapers, including the Philadelphia Inquirer and the Los Angeles Times, where he participated in Pulitzer Prize-winning coverage of the L.A. riots. His work has appeared in Rolling Stone online, the Guardian, and, as a young stringer, the New York Times.